U.S. Pat. No. 3,386,330 of Burris et al. describes an optical rangefinding device previously sold by the former Redfield Company under the trademark ACCU-RANGE®, which includes a two-part reticle system in a variable optical power riflescope. A rear reticle located at a rear focal plane of the scope rearward of an adjustable erector system includes a pair of accurately spaced-apart fixed horizontal stadia wires having a spacing that does not change in the field of view as the erector lens system is adjusted to vary the optical power (image magnification) of the scope. A numeric scale etched in a glass post is inverted in a front focal plane of the scope forward of the power-varying erector lens system. In operation, the image magnification is adjusted until a feature of known size on the target, such as the shoulder to brisket dimension on a deer, just fills the space between the stadia wires. Because the numeric scale is located in the front focal plane, it appears to grow in size and move radially outward in the field of view as the image magnification is increased. The range to the target can be read as the place on the numeric scale at the edge of the field of view where the diaphragm stop of the scope intersects the scale. Due to the image magnification of the erector system, the numeric scale must be very finely etched in glass, which adds to the cost of the riflescope. The present inventors have recognized that great care must be taken during manufacturing to avoid any dirt or debris on the glass post, which would be readily visible under the magnification of the scope.
U.S. Pat. No. 3,684,376 of Lessard, U.S. Pat. No. 4,248,496 of Akin Jr. et al., and U.S. Pat. No. 4,389,791 of Ackerman describe variants of the Burris reticle system. In place of the front focal plane scale used by Burris, the Lessard and Akin Jr. et al. devices include rotating range readouts in the rear focal plane which are coupled to the power adjusting ring of the scope. While they may avoid some of the shortcomings of the Burris reticle system, the devices of Lessard, Akin Jr. et al., and Ackerman introduce different complexities in the rotating readout mechanisms.
The present inventors have identified a need for an improved two-component reticle system for optical rangefinding and holdover estimation.